I managed to recreate the Material from looking at stills in the video. One catch: Sjoerd never shows us exactly what’s going on with those star cubemaps that he’s blending together, so I’m not sure what he did there. In my version, I only use a single cubemap for the starry sky.
It’s a Material in the Material Domain of Post Process (see Details panel under the Material category when you select the main output node).
Here’s the imgur album:
And here’s s showcase vid:
